Para-xylene is an important intermediate in the chemical and fiber industries. Terephthalic acid derived from para-xylene is used to produce polyester fabrics and other articles which are in wide use today.
Usually para-xylene is produced, in a series of steps, from naphtha fractionated from crude oil. Naphtha is hydrotreated and reformed to yield aromatics, which then are fractionated to separate typically benzene, toluene and C8 aromatics comprising xylenes from C9 and heavier aromatics. Toluene and C9 aromatics may be disproportionated to yield additional xylenes. Xylene isomers, with the usual priority being para-xylene, are separated from the mixed C8-aromatics stream using one or a combination of adsorptive separation, crystallization and fractional distillation, with adsorptive separation being most widely used in newer installations for para-xylene production. Other C8 isomers may be isomerized and returned to the separation unit to yield additional para-xylene.
Although low-value light aliphatics such as butanes and butenes offer a substantial theoretical margin for the production of para-xylene, practical processes to effect this conversion have not been available to date. Butane dehydrogenation and dimerization plus aromatization to yield primarily octane isomers is taught in U.S. Pat. Nos. 5,847,252, 5,856,604 and 6,025,533. U.S. Pat. No. 4,367,356 discloses a combination of butene dimerization and alkylation to obtain C8 hydrocarbons. These patents, whose relevant teachings are incorporated herein by reference, do not disclose the production of para-xylene.
In the Journal of Catalysis 1 (1962), pp. 313-328, Pines and Csicsery disclose the aromatization of trimethylpentanes to xylenes, using a nonacidic chromia-alumina catalyst; 2,2,4-trimethylpentane formed only para-xylene. In the proceedings of the 1962 Radioisotopes Physical Science Industrial Process Conference at pages 205-216, Cannings et al. teach dehydrocyclization of 2,2,4-trimethylpentane over a potassium- and cerium-promoted chromia-alumina catalyst to selectively yield para-xylene. British Patent 795,235 teaches the manufacture of para-xylene from 2,4,4-trimethylpentene using a catalyst comprising a Group VI-A oxide, exemplified as a series of chromia-containing catalysts. U.S. Pat. No. 3,202,725 discloses dehydrogenation of isobutane and recycle di-isobutylene using a chromia-alumina catalyst to yield para-xylene and isobutene, plus dimerization of the isobutene using a silica-alumina, phosphoric acid or sulfuric acid catalyst to yield primarily di-isobutylene recycle. U.S. Pat. No. 3,462,505 discloses the dehydrocyclization of 2,2,4-trimethylpentane to yield para-xylene using a catalyst comprising chromia, magnesia and an alkali metal on activated alumina. U.S. Pat. No. 3,766,291 discloses disproportionation of amylene to 2,5-dimethylhexene, which then is selectively converted to para-xylene over a catalyst comprising a Group II metal (exemplified by Zn) aluminate, tin-group metal, and Group VIII metal. U.S. Pat. No. 4,910,357 teaches the aromatization of dimethylhexanes, especially those contained in alkylate, using a catalyst comprising a dehydrogenation metal and a nonacidic crystalline support containing Sn, Tl, In and/or Pb. U.S. Pat. No. 6,177,601 B1 teaches aromatization of 2,5-dimethylhexane to selectively produce para-xylene, using a nonacidic L-zeolite catalyst. U.S. Publication 2004/0044261A1 teaches production of para-xylene from a feedstock rich in C8 isoalkanes or isoalkenes using a catalyst comprising a molecular sieve, Group VIII metal and two or more of Si, Al, P, Ge, Ga and Ti. U.S. Publication 2004/0015026 discloses the manufacture of para-xylene from 2,2,4-trimethylpentane using a catalyst comprising chromium. It should be noted that chromium, as a catalyst constituent, is a toxic element.
None of the above references, drawn to the processing of particular feedstocks, discloses the selective process combination of the present invention. The art heretofore has not taught a practical process for the production of para-xylene from light hydrocarbons.